Mercury cathode electrolysis apparatus



C. DEPREZ March 22, 1955 v MERCURY CATHODE ELECTROLYSIS APPARATUS 4 Sheets-Sheet 1 Filed Nov. 29, 1950 rif/134 IN V EN TOR. Chf/:m55 @5P/Pez A TTONEX' March 22, 1955 c. DEPREZ 2,704,743

MERCURY CATHODE ELECTROLYSIS APPARATUS Filed Nov. 29, 1950 4 Sheets-Sheet 2 0! Q15 P05/rm l p05/ rfa/vf P05/Nm3 l l///lllllllllllllll Mardi 22, 1955 c. DEPRr-:z 2,704,743

MERCURY cATHoDE ELECTRoLysIs APPARATUS Filed Nov. 29, 195o 4 sheets-sheet' s March 22, 1955 c. DEPREZ 2,704,743

MERCURY CATHODE ELECTROLYSIS APPARATUS Filed Nov. 29, 1950 d 4 Sheets-Sheet 4 INVENTOR. CHA/@L ss JEP/Q52 United States Patent O MERCURY CATHODE ELECTROLYSIS APPARATUS Charles Deprez, Brussels, Belgium, assignor to Solvay & Cie, Brussels, Belgium, a company of Belgium Application November 29, 1956, Serial No. 198,199

14 Claims. (Cl. 204-219) This invention is concerned with electrolysis apparatus of the mercury cathode type and, while not limited thereto, is more particularly concerned with apparatus of the character indicated for the electrolysis of aqueous solutions of alakli metal salts, for example, the electrolysis of an aqueous sodium chloride solution.

The electrolysis of sodium chloride in aqueous solutions to liberate chlorine and sodium is carried out in various types of electrolysis apparatus. The mercury cathode electrolytic cell has been found particularly effective for carrying out this reaction and is extensively used in commercial electrolysis installations. In the mercury cathode cell, in which the salt solution to be decomposed is continuously circulated, a stream of metalic mercury is caused to ow through the cell, generally over the bottom thereof, in contact with the brine. As metallic sodium is liberated during the electrolysis reaction, it combines with the flowing mercury stream to form an amalgam therewith and the thus-formed amalgam is removed from the electrolysis cell and later decomposed in a denuding cell to liberate mercury, which is recycled to the electrolysis cell, and to form sodium hydroxide and hydrogen by reaction of the liberated metallic sodium with water. The chlorine liberated in the electrolysis cell is removed in any convenient manner. The anodes of the electrolysis cell are generally formed from graphite and are electrically connected to a source of electrical current. ln the most satisfactory commercial cells each graphite anode is in the form of a variously-shaped block having a substantially planar lower surface which is disposed at a predetermined distance from the surface of the owing mercury cathode. This distance is referred to as the anode-cathode gap, and the voltage in the cell varies, of course, with the size of the gap. During normal operation of the cell, each graphite anode is gradually worn away and this wear is generally localized at the lower surface of the anode. Since the ow of mercury remains substantially constant, wearing away of the lower surface of the anodes gradually increases the anode-cathode gap and this increase in the size of the gap causes a gradual increase in the voltage. A certain amount of voltage increase can be tolerated but after it has reached a maximum practical value, which depends upon the particular cell, further increase in voltage must be prevented if economical operation of the cell is to be achieved. Accordingly, it has been customary to lower the anodes from time to time to reduce the anode-cathode gap to the optimum distance, e. g. the distance at which each anode was originally set when new. While such operation does, of course, compensate for the wear of the anodes and keeps the voltage within practicable operating limits, this method has certain practical disadvantages. The graphite anodes have a stem or support extending upwardly through the cover plate of the cell and to permit the lowering of the anodes to compensate for wear, as just described, it is necessary to provide for sliding movement of the anode stem in the cover plate. This makes necessary the provision of special gasket means which will permit the sliding movement and yet which will retain the fluid-tightness of the cell. From a practical standpoint this presents many problems and is not wholly satisfactory. Furthermore, as the anode blocks wear away, more of the stem is moved into the cell and the upper surface of each anode gradually moves downwardly below the surface of the brine. Since, for efficient oper- 2,704,743 Patented Mar. 22, 1955 ICC ation, it is desirable to prevent flow of brine across the top of the anodes and to force as much as possible of the brine to flow between the mercury cathode and the lower surface of each anode, i. e. through the anodecathode gap, the gradual lowering of the upper surface of each anode nulliies the damming effect which the anodes have upon the ow of brine and permits gradually increasing quantities of the brine to flow through the cell without passing through the anode-cathode gap or coming in contact with the anodes, and thus avoiding electrolytic decomposition. In accordance with present practice, therefore, the operator has the choice either of lowering each anode and using as much as possible of the block but permitting gradually increasing ow of brine across the top of the block, or of replacing the anodes after only a slight amount of wear has occurred. Either choice is obviously disadvantageous from the standpoint of economical elliciency of operation. Furthermore, with this procedure all of the anodes require replacement at substantially the same time which necessitates the provision of a large labor force for a relatively short period of time. Various other proposals have been made to solve the problem of compensating for anode wear, but all suffer from one or more practical disadvantages and generally involve a complicated and expensive construction.

It is the principal object of the invention to provide an improved mercury cathode electrolysis apparatus which avoids the disadvantages of prior apparatus;

It is another object of the invention to provide a mercury cathode electrolysis cell wherein full utilization of the anodes is possible without lowering the eliciency of the electrolysis reaction;

It is another object of the invention to provide an apparatus of the character indicated wherein the adjustment of the anode-cathode gap to compensate for wear of the anodes can be carried out rapidly and eliiciently.

According to the invention, l provide an electrolysis apparatus of the mercury cathode type which is formed with gradually converging longitudinal bottom and top surfaces, whereby the distance between the bottom or sole of the cell and the top of the cell is substantially greater at one end of the cell than it is at the other and the vertical space in the cell in which the anodes are disposed gradually decreases progressively from one end of the cell to the other. The cell of my invention is provided with a plurality of individual cover plates, each of which supports one or more anodes. The anodes are of gradually decreasing height progressively from the deeper end to the shallow end of the cell. As the anodes in one of the cover plates become worn and the anodecathode gap reaches the maximum practical limit, the cover plate with the anode supported by it is moved toward the shallow end of the cell. In the new position of the cover plate the distance between the cover plate and the sole of the cell is less, and consequently the anode-cathode gap will be less. The face of the anode will in etect have been lowered with respect to the flowing mercury cathode and will provide an anodecathode gap within the desired operating range. In the preferred embodiment of my apparatus, each cover plate is adapted to be moved longitudinally of the cell a distance substantially equal to the length of the cover plate. That is, each plate is adapted to be moved into the position formerly occupied by the next adjacent cover plate. Dur1ng the cover plate shift, the cover plate at the shallow end of the cell is removed from service, all of the remaining cover plates are moved one position toward the shallow end, and a cover plate supporting a new anode is installed at the deep end of the cell. The distance between the sole and the top wall or cover plate at the shallow end of the cell is determined by the height of the minimum practical anode, i. e. the height of an anode element which has been worn away to the maximum perm1ss1ble extent, and the height of the deep end of the cell is determined by the height of the fresh anode element used in the cell. These two heights determine the combined slope of the converging top and bottom. surfaces of the cell. Thus, if m and n represent, respectively, .the slopes of the top and bottom walls of the cell 1n umts u per 1000 units u of cell length, L equals the cell length in units u, and D represents the difference in height in units u of the anodes at each end of the cell at any given time, then,

It will be apparent that in my apparatus the relationship between the top surface of each anode and the cover plate remains constant, since the anodes are iixedly mounted in the cover plates and remain so during their entire passage through the cell, and the anodes can therefore be maintained at all times above the surface of the brine, whereby to prevent the ow of the brine across the top of the anodes. The wearing away of the bottom surface of the anodes does not in any way change their relative relationship to the cover plates.

Anode blocks of the type available commercially are compressed into shape during their manufacture and the portions of the block adjacent the faces tend to have a greater density than the core portion of the block. I have found that, as a result of this difference in density, wear of the anode occurs at a substantially constant rate until the denser portion of the block face opposite the mercury cathode has been worn away, and then the rate of wear gradually increases as the less dense portion of the block is exposed. In order to compensate for this difference in rate of wear, in a preferred embodiment of my invention, I form the bottom wall or sole of my cell as a substantially rectilinear surface but form the top Wall of the cell as a slightly curved surface, of gradually varying curvature, for example, gradually increasing in curvature progressively toward the electrolyte inlet end of the cell in a co-current cell.

The curvature of the top surface, i. e. the surface of the cell defined by the cover plates, is, of course, determined by suitable formation of the top edge of the side walls of the cell upon which the cover plates rest.

Other features and objects of my invention will be apparent from the following detailed description of illustrative embodiments thereof, and from the drawings, wherein,

Fig. l is a longitudinal sectional view of a mercury cathode electrolysis cell embodying features of the present invention:

Fig. 2 is a plan view of the cell shown in Fig. 1;

Fig. 3 is a transverse sectional view taken approximately along the line 3-3 of Fig. 2;

Fig. 4 s a plan view of a modified form of the embodiment of Fig. 1;

Fig. 5 is a longitudinal sectional view taken approximately along the line 5-5 of Fig. 4;

Fig. 6 is an enlarged view of a portion of Fig. 5 showing details of the mounting arrangement for the anode;

Fig. 7 is a transverse sectional view taken approximately along the line 7--7 of Fig. 4;

Fig. 8 is a sectional view taken approximately along the line 8 8 of Fig. 7;

Fig. 9 is a side elevation of the removable partitions shown `in Fig. 5;

Fig. l0 is a plan view of another modification of the embodiment of Fig. 1;

Fig. l1 is a longitudinal sectional view of another embodiment of the invention;

Fig. l2 is a similar view of another embodiment of the invention having a curved upper surface.

Fig. 13 is a similar view of still another embodiment of the invention combining the features of the embodiments of Fig. 1l and l2;

Fig. 14 is a diagrammatic view of an embodiment of the invention having a curved top wall;

Fig. 15 is a similar view of a modified form of the embodiment of Fig. 14;

Fig. 16 is a similar view of another modification of the embodiment of Fig. 14, and

Fig. 17 is a similar view of still a further modified form of the embodiment of Fig. 14.

Referring to the drawings, and particularly to Figs. l, 2 and 3, the numeral 20 designates generally a mercury cathode electrolysis cell. In order to simplify the dev scription of the cell, the slope of the converging top and bottom walls of the cell has been exaggerated and the slope of the bottom surface of the anodes has been correspondingly exaggerated. In practice, mercury cathode electrolytic cells are generally about 30 to 75 feet in length, whereas the difference in height between th@ Shallow end and the deep end of an elongated cell of this type embodying features of my invention will ordinarily be of the order of 1 to 4 inches, and the cell may have as many as 5 to 30 anode-supporting cover plates. It will be obvious, therefore, that the slopes of the converging top and bottom walls of the cell will, in practice, not be as readily apparent as they are in the cell in the drawings. The cell illustrated is, however, taking into account the exaggerated slopes as mentioned, a practical embodiment and is operable in accordance with the invention.

The cell 20 comprises a bottom or sole 22 which is formed from iron or other amalgamable metal, end walls 23, 24, and side walls 25 which are conveniently formed from channel irons secured along their lower faces, as by bolting to the sole 22, suitable gaskets being placed between the opposed surfaces to insure fluid-tightness. The top faces of the walls 25 are provided with upwardly extending ridges or anges 26. Cell 20 has a gas outlet 27 at its shallow end, i. e. at the right of Fig. l, the gas outlet 27 being formed in an end cover plate 28, and a gas outlet 29 at its deep end formed in a cover member adjacent end cover plate 30. The shallow end is also provided with a mercury inlet 31 provided in the sole plate 22 and the opposite end of the cell is provided with an amalgam outlet 32 which may be suitably connected to a denuding cell (not shown) for decomposition of the amalgam to free the amalgam of sodium and to liberate mercury for recirculation to cell 20 through inlet 31. Brine inlets and outlets are provided at 34 and 35 at each end of the cell in one of the side walls 25 at the level at which the brine is to be maintained in the cell. The cell shown in Fig. 1 is adapted for co-current iiow of brine with respect to mercury, the brine entering the cell through inlet 35 and leaving the cell through brine outlet 34. A bale 37 extends downwardly from end cover plate 30 adjacent brine outlet 34 in order to prevent chlorine from escaping or becoming diluted by air when cover plate 30 is removed to permit cleaning of the deep end of the cell from time to time to remove impurities which accumulate on the surface of the amalgam.

Resting on the top surface of the side walls 25 are cover plates 45, 46, 47, 48 and 49. As will be seen by reference to Fig. 1, the cover plates are not in direct abutting relationship but are separated by a small space or clearance. This space between the plates and the space between the sides of the plates and the anges 26 on the upper surfaces of side walls 25 are conveniently sealed by means of a gasket 50 which is suitably held in position by a pressure member 51 of any convenient construction. If desired, luting or other removable sealing arrangement may be provided. The inner surfaces of the anode cover plates and the side and end walls of the cell 20 are covered with rubber or other like corrosion-resistant, non-conductive material for protecting these surfaces from chemical attack by brine and chlorine. Supported from the cover plate 45 is the anode 55, which, in the embodiment illustrated, comprises a substantially parallelepipedal block of compressed graphite. The anode 55 illustrated in Fig. l has a stem arrangement, designated generally by the numeral 60, which extends through an aperture in cover plate 45 and is suitably sealed, as by a resilient gasket 62 which is pressed downwardly into sealing engagement by a follower member 63 by means of bolts 64 which threadedly engage cover plate 45.

Each of the cover plates 46, 47, 48 and 49 is similarly provided with anodes 56, 57, 58 and 59, respectively, having stem arrangements 65, 66, 67 and 68, respectively. The anodes are all arranged at a predetermined distance from the cover plates, the distance being chosen to permit the top surfaces of the anodes to extend slightly above the surface of the brine in the cell 20, i. e. above the level determined by the brine inlets and outlets 34 and 35. The lower surface of each anode is arranged at a predetermined distance from the sole 22. Owing to the converging slope of the sole 22 and the top wall of the cell formed by the cover plates, the anodes are of gradually decreasing height as they progress from the deep end to the shallow end of the cell 20. As previously mentioned, in the embodiment illustrated in Fig. l the slopes of the top and bottom of the cell have been purposely exaggerated in order clearly to show the construction of the invention. In practice, the slope of the bottom surface of the anodes is less obvious. As shown in Fig. 2, current is supplied to the cell 20 through the bus-bars 70, 71,

72, 73, and 74 which are connected in parallel to a main bus-bar 76 and lead to anodes 55, 56, 57, 58 and 59, respectively. Bus-bar 76 is connected to any conven ient source of direct current in accordance with standard prilgtice in the operation of mercury cathode electrolytic ce Advantageously, and in accordance with the preferred embodiment of my invention, the mounting arrangement for the anodes comprises the structure shown in the embodiment of Figs. 4 to 8. Figs. 4 to 8 show a modied form of the embodiment of Figs. 1 to 3, like parts being designated by like numbers with the addition of 100.

The cell 120 has a sole 122, end walls 123, 124, side walls 125, a mercury inlet 131, an amalgam outlet 132, a brine inlet 134, a brine outlet 135 and gas outlets 127 and 129. For reasons which will be apparent as the description proceeds, the gas outlets 127 and 129 are adapted to discharge either into a concentrated gas header 127a or a dilute gas header 129a, the flow of gas being controlled by suitable valves (not shown).

Referring particularly to Figs. 4 and 5, it will be seen that each of the cover plates supports nine anodes instead of the single anode of the embodiment of Fig. 1. Thus, shallow end cover plate 145 supports nine anodes 155 arranged in three rows of three each. Referring more particularly to Fig. 6, the stem arrangement of each of the anodes 155 comprises a graphite rod 180 which is press-fitted into an aperture in the top surface of anode 155 and has a threaded core 181 in which a threaded copper rod 183 is fitted in electrically conductive relationship. The rod 183 has an upper enlarged portion 184 which is connected by means of bolts 185 to busbar 170 which carries the current to the anode. The inner surface of the cover plate 145 is coated with a layer 190 of a corrosion resistant non-conductive material, such as rubber, which extends through the aperture 192 in cover plate 145, and overlies a portion of the top surface of cover plate 145, thus forming a gasket corresponding to the gasket 62 shown in Fig. l. Since the copper rod 183 must be electrically insulated from the cell body, it is necessary to enclose it with insulating material. For this purpose l provide an insulating sleeve 193 which surrounds the copper rod 183. A gasket washer 194 provides gas tightness for the cell around the anode stem. The various parts of the assembly are pressed in Huid-tight engagement by a pressure member comprising a nut 196 threadedly engaged on copper rod 183 which presses a metal washer 197 and an insulating gasket washer 195 against the rubber coating lying on the top surface of the cover plate. The anodes 156, 157, 158 and 159 are similarly mounted in cover plates 146, 147, 148 and 149, respectively, and are supplied with current rom bus-bars 171, 172, 173 and 174, respectively, the bus-bars being connected in parallel to a main bus-bar 176 which is connected to the source of direct current.

Adjustment of the cathode-anode gap, in accordance with the invention, is carried out as follows, referring particularly to Fig. 5, the space normally occupied by the end cover plate 145 being designated position i and the space occupied by adjacent cover plate 146 and 147 being designated as position 2 and position 3, respectively. When the anode shift is to be made, cover plate 145 with its depending anodes 155 is removed from the cell, cover plate 146 is moved from position 2 to position l and all of the other cover plates are similarly moved one position toward the shallow end of the cell, a cover plate carrying a set of new anodes being added at the deep end of the cell. Since the shift of anodes may be carried out by moving one cover plate at a time, while the remainder of the cell continues in operation, in order to avoid undue dilution of the gas evolved by an influx of air l advantageously employ temporary partitions as shown in Fig. 9. The partition 210 has a main body portion 211 which is adapted to extend downwardly into the cell between the side walls 125 and has wing portions 212 which are adapted to rest upon the top surfaces of the side walls 125 between the edge flanges. These temporary partitions are made from any convenient relatively thin material such as plastic, plywood, or other material not susceptible to corrosion by the brine and chlorine. When the anode shift is to be made, the gaskets 150 are removed from cover plates 145 and 146 and a temporary partition 210 is inserted between cover plates 145 and 146 and between cover plates 146 and 147. The bus-bars 170 are disconnected from the rods 183 and cover plate 145 is removed. At this time the valves'in gas outlets 127 and 129 which, during normal operanon of the cell, conduct the gas into the concentrated gas header 1270, are changed to conduct the gas into the dilute gas header 129:1. While some dilution of the gas occurs, it is possible by means of the partitions 210 to prevent undue dilution so that a usable diluted gas is evolved during the anode change-over penod.- After the removal of cover plate 145, cover plate 1,46 1s moved from position 2 to position l, i. e. the posltion previously occupied by cover plate 145. This is the arrangement shown in Fig. 5. It will be noted that the anodes 156 are close to the cathode surface whereas the anodes 157 (which have not yet been moved) are spaced a substantial distance from the cathode. The change in the cathode-anode gap by the shift is thus read- 1ly apparent. When upon subsequent shifting of the cover plates, cover plate 147 is movedV into position 2, the anodes 157 will be at the same distance from the cathode as are the anodes 156. After each cover plate has been shifted into its new position, the temporary partitions 210 are removed, the gaskets 150 replaced, the bus-bars connected and the anodes place back in operation. Since the cell has a gas outlet at each end, it is not necessary to discontinue operation of the cell while the other cover plates are being shifted. While the cover plate shift is advantageously carried out by moving one cover plate at a time, as just described, the shift may also be carried out by moving all of the anode plates at the same time. In this case, all of the bus-bars would be disconnected, cover plate removed, and then the remaining cover plates moved in a body one posltion toward the shallow end. New anodes would then be supplied at the deep end, the bus-bars connected, and the cell placed back in operation. f'

As previously mentioned, the decreaseV in vertical distance between the bottom and top walls -of the cell from one end of the cell to the other, corresponds to the decrease in thickness of the anode element, e. g. the anode block illustrated, as it moves from the deep end to the shallow end of thre cell. This decrease in thickness can be designated E-e, where E'is the thickness of the new anode block and e is the thickness of the block which has been Worn away to the maximum practical extent, i. e. when removed from the cell.

To illustrate the invention in terms of a commercial embodiment, a cell 585 inches long and 40 inches wide is provided with fifteen equal-sized cover plates, each supporting nine anodes. Each cover plate will be approximately 39 inches long and 45 inches wide and each anode will have a lower face 12% x 12% inches. Using the formula previously described, and designating T the time between two adjustments of the anode-cathode gap, and V the rate of Wear of the graphite at the moment the anode at the shallow end of the cell is removed, the anode at the deep end of the cell, which will have a life of T, will have a thickness of E-T V. Thus,

m-l-n For optimum flow of mercury and amalgam across the bottom of the cell, the slope n of the bottom will be approximately 3 inches per 1,000 inches of cell length and e will be about 0.75 inch. The Equation 1 then be- Each shift of the anode plates is carried out in such a way that the change of the anode-cathode distance will correspond to the wear in the time period T. Thus of 2.73 amperes per square inch is about 0.0117 inch per day.

When E=4 inches and e=% inch, the total wear will be 3.25 inches. Then the total life of the anodes will be:

3.25:0.0l17=278 days During that time, each anode plate will remain T days in each of the 15 positions in the cell. So

and

T=(278:l)=181/2 days During that time, the anode wear will be TV inches or 181/2 X0.0ll7=0.217 inch Using an optimum anode-cathode gap of 0. 2 inch, the spacing obtained after each shift, after a period of 181/2 days of operation, the gap will be 0.2-|-0.217==0.417 inch and the average anode-cathode gap in the cell will be 0.2-l-0.417=0.617=0 3085 inch If for any reason the total current passing through the cell, that is the current density, is increased or decreased, the speed of graphite wear will be greater or less, and the time period T between two adjustments of the cathodeanode distance will be shortened or increased.

As we have already seen 1o00(m+)=TV In our case TV=0.217 inch thus 0.217 m-l-n-minches/1000 meh 12.75X -3X (m-l-n) inches that is to say, if (m-|n)=5.66 inches per 1000 inches, 12.75Xl03 5.66=0.0722 inch At the end of the cell, when a new anode with a lower surface parallel to the upper surface is placed with an anode-cathode gap 0.2 inch at the center, it will have respectively at both ends an anode-cathode gap of 0.1639 inch and 0.2361 inch. This diterence, however, will be rapidly worn away by the difference in current density on both ends of the anodes. When an anode plate supports more than one anode, as in the embodiment of Figs. 4-8, the anode gap difference may be too great and may necessitate shaping the anodes so that the lower surface will have a slope of (m-i-n) inches per 1000 inches with respect to the upper surface of the anode.

Alternatively, I may space the three transverse rows of anodes at slightly varying distances from the cover plate. This arrangement is shown in Fig. 4. In this way all new anodes can be identical in size and need not be specially treated or machined. Thus, using the values of m and n employed above and designating as H' the distance from the cover plate to the top of the first anode 155 at the left of Fig. 4, the distance H2 from the cover plate to the second anode will be greater than H' by 13 X 10-3X5.66=0.0735 inch H2=H+0.0735 inch The distance H3 of the third anodes from the cover plate will be greater than the distance H2 by 0.0735 inch.

With this arrangement, the dierence in thickness from 8 one anode to another will be 0.0722 inch which is satisfactory for operation.

Fig. 10 shows a slightly modified form of the arrangement of Figs. 4-8 wherein each cover plate supports four anodes arranged in a square, the four anodes being connected in parallel to the bus-bar 218 by suitable conductors 219. Instead of arranging the cover plates in substantially co-planar relationship as in the embodiments shown in Figs. l10 just described, I may arrange the cover plates in step-Wise fashion as shown in Fig. 11, wherein each cover plate is parallel to the sole of the cell but disposed at a slightly diiferent level with respect to the adjacent cover plates. Thus, in the cell 220 shown in Fig. 1l the side Walls 225 are formed with a stepped upper edge progressing downwardly from the deep end to the shallow end of the cell. The cover plates 245, 246, 247, 248 and 249 which rest upon the walls 225 are similarly disposed in step-wise fashion parallel to the sole 222 and the upper and lower surfaces of the anodes 255, 256, 257, 258 and 259 are likewise parallel. In order to insure fluid-tightness between the cover plates, each plate is provided with a transverse liange. Thus, the cover plate 245 has a flange 245a which engages a depending ange 230a in end cover plate 230. Cover plates 246, 247, 248 and 249 are provided with flanges identical with flange 245a. While in the diagrammatic view shown in Fig. ll these flanges appear somewhat exaggerated in height, in practice the anges are suliiciently small that effective sealing can be obtained by means of luting or gaskets.

In the embodiments of my invention described above, the top surface of the cell defined by the cover plates resting upon the top edges of the side walls is substantially rectilinear. As previously mentioned, however, I may advantageously form the side walls of my cell with a slightly curved top surface in order to compensate for the somewhat increased rate of wear of the anodes as they progress toward the shallow end of the cell, owing to the wearing away of the more dense outer portion of the anode block, as described above. Furthermore, I have found that in practice the differences in the concentration of the brine in different portions of the cell and variations in current density cause slight variations in the rate of wear of the anodes in diierent parts of the cell. In order to compensate for the combined eect of these variations in the rate of wear and to thus obtain an optimum adjustment of the anode-cathode gap at all times, I provide an appropriate curvature along the top edge of the side walls of the cell. Thus, referring to Fig. 12 wherein I employ the numerals used in Fig. 1 with the addition of 300 to designate like parts, the cell 320 has a sole 322 and side walls 325 on the top surface of which are supported cover plates 345, 346, 347, 348 and 349. It will be observed that the sole 322 is rectilinear but that the upper edge of the side walls 325 curves slightly as it slopes from the deep end to the shallow end of the cell, the degree of curvature increasing as the shallow end of the cell is approached. As in the case of the embodiment shown in Fig. l, the curvature has been exaggerated in order to show more clearly the desired arrangement. Since in practice, as previously indicated, a cell of 585 inches length will have a difference in height of less than 4 inches, it will be obvious that the curvature will not be as readily apparent as it is in Fig. 12. The arrangement of Fig. 12 shows a cell adapted for co-current W of brine and mercury. As previously mentioned, various factors influence the rate of wear of the anodes and the curvature of the top surface of the cell may be varied to cornpensate for these factors. Thus, referring to Figs. 14 and l5 there are shown diagrammatically two possible variations in the curvature of the side walls of a cell in which co-current tlow is employed. In the arrangement shown in Fig. 14 the wall slopes downwardly with decreasing curvature toward the brine and mercury inlet end of the cell, whereas in the arrangement of Fig. 15 the top surface of the cell slopes downwardly with increasing curvature toward the brine and amalgam outlet end of the cell. Obviously, the anodes will progress toward the right in the cell of Fig. 14 and to the left in the cell of Fig. 15. Figs. 16 and 17 show diagrammatically similar cell arrangements embodying countercurrent brine and mercury ow. In Figs. 14, 15, 16, and 17 the arrows A indicate the direction of movement of the cover plates during the cover plate shift. The arrows B show the direction of iiow of the electrolyte andthe arrows C indicate the direction of ow of the mercury cathode.

It will be apparent that the precise curvature of the top surface of any given cell will depend upon the method by which the cell is to be operated and the nature of the anodes to be used in it. The calculations set forth above are concerned with an embodiment having substantially rectilinear top and bottom surfaces. The decrease of curvature can be readily calculated for any given set of conditions, using the type of calculation set forth above. It will be understood that my cell is adapted to be operated in accordance with usual commercial practice using the current densities and brine concentrations commonly employed.

In another embodiment, the curved surface of Fig. l2 can be combined with the stepped arrangement of Fig. 11 to provide an arrangement of the type shown in Fig. 13. In this arrangement the cell 420 is provided with side walls 425 having a top surface of stepped outline, the steps being of varying heights.

While I have thus shown and described illustrative embodiments of my invention, it will be obvious to those skilled in the art that various changes and modications may be made without departing from the scope of the invention as defined in the appended claims and it is intended, therefore, that all matter contained in the foregoing description and in the drawings shall be interpreted as illustrative and not in a limiting sense.

What I claim and desire to secure by Letters Patent is:

l. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of individually-removable cover plates cooperating with the top edges of said side walls to close said cell body, and at least one anode depending from each of said cover plates, each anode having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body.

2. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of individually-removable cover plates cooperating with the top edges of said side walls to close said cell body, and a plurality of anodes depending from each cover plate, said anodes having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, and the anodes decreasing in Vertical thickness from said deep end to said shallow end of said cell body.

3. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of similar individually-removable cover plates cooperating with the top edges of said side walls, and at least one anode depending from each of said cover plates, each anode having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from one end of said cell body to the other end of said body and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, the anodes depending from said plates being of different vertical thickness from the anodes depending froin adjacent plates and the anodes depending from the several plates varying progressively in vertical thickness from one end of the cell to the other.

4. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of individuallyqemovable cover plates cooperating with the top edges of said side walls to close said cell body, and at least one anode depending from each of said cover plates, each anode having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from a deep'end of said cell body to a shallow end of said body, and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, a mercury inlet at said shallow end, and an amalgam outlet at said deep end, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body.

5. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of individually-removable cover plates cooperating with the top edges of said side walls to close said cell body, and a plurality of anodes depending from each cover plate, said anodes having an active bottom surface substantially parallel to said planar bottom and the distance between the top surfaces of said anodes and said cover plates being substantially constant, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body.

6. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, a plurality of individually-removable cover plates resting upon the top edges of said side walls, and at least one anode depending from each of said cover plates, each anode having an active bottom surface substantially parallel to said planar bottom, said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom and said top edges being formed with a stepped outline, the longitudinal portions of said top edges being substantially parallel to the bottom of the cell, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body.

7. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, a plurality of individually-removable cover plates resting upon the top edges of said side walls, and at least one anode depending from each of said cover plates, each anode having an active bottom surface substantially parallel to said planar bottom, said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, said top edges being of continuous unidirectionally-varying slope, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body.

8. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, a plurality of individually-removable cover plates resting upon the top edges of said side walls, and at least one anode depending from each of said cover plates, each anode having an active bottom surface substantially parallel to said planar bottom, said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, said top edges being of continuous unidirectionally-varying slope, the anodes depending from said plates being of diierent vertical thickness from the anodes depending from adjacent plates and the anodes depending from the several plates decreasing progressively in vertical thickness from said deep end to said shallow end.

9. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, a plurality of individually-removable cover plates resting upon the top edges of said side walls, and at least one anode depending from each of said cover plates, each anode having an active bottom surface substantially 1 1 parallel to said planar bottom, said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, said top edges gradually increasing in slope progressively toward the shallow en d of the cell, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body.

10. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of individually-removable cover plates cooperating with the top edges of side walls to close said cell body, and at least one anode depending from each of said cover plates, an electrolyte inlet at one end of said cell, an electrolyte outlet at the other end of said cell, said electrolyte outlet being arranged to determine the level of the electrolyte in said cell, eachanode having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, and the anodes decreasing in vertical thickness from said deep end to vsaid shallow end of said cell body.

1l. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walis and end walls, a plurality of individually-removable cover plates cooperating with'the top edges of said side walls to close said cell body, and a plurality of anodes depending from each cover plate, and the anodes on each cover plate being arranged in rows, an electrolyte inlet at one end of said cell, an electrolyte outlet at the other end of said cell, said electrolyte outlet being arranged to determine the level of the electrolyte in said cell, said anodes having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, and and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, said top edges being of continuous unidirectionally-varying slope, and said anodes decreasing in vertical thickgesis from said deep end to said shallow end of said cell 12. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of individually-removable cover plates cooperating with the top edges of said side walls to close said cell body, and at least one anode depending from each of said cover plates, an electrolyte inlet at one end of said cell, an electrolyte outlet at the other end of said cell, said electrolyte outlet being arranged to determine the level of the electrolyte in said cell, each anode having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which convergesl with the plane of said bottom, said top edges being of continuous unidirectionally-varying slope, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body.

13. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of individually-removable cover plates cooperating with the top edges of said side walls to close said cell body, and at least one anode depending from each of said cover plates, each anode having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, and said bottom and said top edges of said side walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body, and an opening at said shallow end and at said deep end for circulation of brine through said cell, said openings being arranged to determine the level of the electrolyte in said cell.

14. A mercury cathode electrolysis cell, comprising an elongated cell body having a planar bottom, side walls and end walls, a plurality of individually-removable cover plates cooperating with the top edges of said side walls to close said cell body, and a plurality of anodes depending from each cover plate, and the anodes on each cover plate being arranged in rows, said anodes having an active bottom surface substantially parallel to said planar bottom, said side walls decreasing in height from a deep end of said cell body to a shallow end of said body, and said bottom and said top edges of said side Walls converging toward said shallow end, whereby corresponding points in said cover plates lie on a line which converges with the plane of said bottom, and the anodes decreasing in vertical thickness from said deep end to said shallow end of said cell body, and an opening at said shallow end and at said deep end for circulation of brine through said cell, said openings being arranged to determine the level of the electrolyte in said cell.

References Cited in the file of this patent UNITED STATES PATENTS 678,526 Stewart July 16, 1901 2,232,128 Mueller Feb. 18, 1941 2,312,452 Taylerson Mar. 2, 1943 2,334,354 Richardson Nov. 16, 1943 2,399,254 Rieger Apr. 30, 1946 2,467,892 Horst Apr. 19, 1949 2,503,337 Hirsh Apr. 1l, 1950 FOREIGN PATENTS 334,332 France Oct. 20, 1903 238,997 Switzerland Sept. 15, 1945 

1. A MERCURY CATHODE ELECTROLYSIS CELL, COMPRISING AN ELONGATED CELL BODY HAVING A PLANAR BOTTOM, SIDE WALLS AND END WALLS, A PLURALITY OF INDIVIDUALLY-REMOVABLE COVER PLATES COOPERATING WITH THE TOP EDGES OF SAID SIDE WALLS TO CLOSE SAID CELL BODY, AND AT LEAST ONE ANODE DEPENDING FROM EACH OF SAID COVER PLATES, EACH ANODE HAVING AN ACTIVE BOTTOM SURFACE SUBSTANTIALLY PARALLEL TO SAID PLANAR BOTTOM, SAID SIDE WALLS DECREASING IN HEIGHT FROM A DEEP END OF SAID CELL BODY TO A SHALLOW END OF SAID BODY, AND SAID BOTTOM AND SAID TOP EDGES OF SAID SIDE WALLS CONVERGING TOWARD SAID SHALLOW END, WHEREBY CORRESPONDING POINTS IN SAID COVER PLATES LIE ON A LINE WHICH CONVERGES WITH THE PLANE OF SAID BOTTOM, AND THE ANODES DECREASING IN VERTICAL THICKNESS FROM SAID DEEP END TO SAID SHALLOW END OF SAID CELL BODY. 